Read How Chemists Proved Werner's Structures
Werner's coordination theory wasn't guessed — it was proved with two simple experiments: how many ions a solution's conductance implies, and how much AgCl precipitates instantly with AgNO₃. Learn to read both.
Determining Structure from Experimental Evidence is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Conductance tells you how many ions form
Treating CrCl3·6H₂O with excess AgNO₃ gives 3 mol of AgCl, meaning all three chlorides are ionisable — so the correct formula places every water inside the coordination sphere and every chloride outside it: [Cr(H₂O)₆]Cl₃. Choose D. The same logic ranks a conductance series: more ionisable chloride in a PtCl4·nNH₃ series means higher measured conductance.
Only some chlorides precipitate — that's the clue
If AgNO₃ precipitates only a fraction of the total chlorine instantly, the rest must be coordinated (bonded to the metal, not free). When 1/3 of CrCl3·6H₂O's chlorine precipitates, two of the three chlorides are still attached to Cr, giving [CrCl2(H₂O)₄]Cl·2H₂O. The same reasoning applies to CoBr₂Cl·4NH₃: if AgNO₃ precipitates AgCl instantly but not AgBr, the chloride is the free counter-ion and both bromides stay coordinated — [CoBr2(NH₃)₄]Cl.
From ion count to coordination number and oxidation number
In [Pt(O₂)(en)₂Br]2+, work the coordination number and platinum's oxidation number from charge balance across every ligand (peroxide, two neutral bidentate en, one Br⁻) rather than from a memorised default oxidation state for platinum.
OlympIQ exam lens
- ✓Ions that precipitate instantly with AgNO₃ are the free counter-ions; unprecipitated chlorine is coordinated.
- ✓More free (ionisable) ions means higher solution conductance.
- ✓Total conductance-derived ion count = coordination sphere charge + number of counter-ions, not the total atom count in the formula.
- ✓Always rebuild oxidation number from overall charge balance across all ligands, not from a memorised 'default' oxidation state.
Don't assume the formula as written shows the true coordination sphere — always re-derive it from the conductance/precipitation data given, since Werner-era formulas like 'CrCl3·6H₂O' deliberately hide which atoms are actually bonded to the metal.
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